Sequential colour display is conventionally used in projectors to display a colour image with a single display cell valve. FIG. 1 shows the division of one display period of a video image, called a video frame, into a plurality of time segments each assigned to one colour component of the image. In this example of subdivision, the video frame includes three time segments for each colour component, i.e. three segments R1, R2, R3 for the red component, three segments G1, G2, G3 for the green component, and three segments B1, B2, B3 for the blue component. Each red time segment is followed by a green time segment and then a blue time segment. These segments generally have substantially identical durations.
With this kind of video frame structure, the colour components of the video signal of the image are displayed sequentially one after the other, the colours of the pixels of the image being restored thereafter by an effect of integration (in the human eye). The use of a plurality of time segments for each colour component enables the emission of light to be distributed over the whole duration of the frame and to group closely together the emissions of red, green and blue light. This reduces the phenomenon of colour break-up perceived by the human eye when the emissions of different colours are far apart, for example when there is an average red emission at the beginning of the frame, an average green emission in the middle of the frame and an average blue emission at the end of the frame.
In the case of micromirror (DMD) projectors using binary planes to address the micromirror cells, the video frame portion associated with each colour component is divided into sub-periods, usually called sub-scans, having different durations and distributed in the various time segments associated with said colour. One solution for limiting colour break-up even further in this type of projector is to divide the sub-scans of longer duration into sub-scans of shorter duration and to distribute them over all the time segments of the frame to spread the emission of light even further. This results in emission of light that is distributed almost uniformly between the time segments for a given colour.
In the case of LCOS or OLED valve projectors, the cells of the valve are generally addressed segment by segment using voltage control or current control to emit or to pass the same quantity of light during each segment of a given colour.
FIG. 2 shows the distribution of light emitted for a video level 128 to be displayed using an LCOS or OLED valve and a uniform distribution between the segments. The same quantity of light is emitted during each of the time segments. This results in a light emission distribution almost identical to that of DMD valve projectors when the more significant sub-scans are distributed over all the time segments of the frame.
The drawback of this spreading of light is that it generates a blurring effect in the images displayed.